1. Field of the Invention
The present invention relates to a titanium part made of either titanium or an alloy thereof, and more particularly, the present invention relates to a titanium part of an internal combustion engine that is exposed to the combustion gas of the internal combustion engine.
2. Description of the Related Art
Recently, titanium or an alloy thereof (which will be collectively referred to herein as a “titanium material”) has been used more and more often as a material for an engine part in order to increase the output of an engine and decrease its weight.
Generally speaking, a titanium material is a lightweight material that has high mechanical strength but low abrasion resistance. That is why a titanium part that should have high abrasion resistance is sometimes subjected to an oxidation treatment. If a titanium part is processed by an oxidation treatment, a hard titanium oxide film is formed on the surface of the titanium part, thus increasing the abrasion resistance of the titanium part. However, since titanium oxide is brittle, the titanium part with the titanium oxide film has decreased fatigue strength and decreased impact strength. The fatigue strength and impact strength of a titanium part can be increased to a certain degree by thickening the part. In that case, however, the weight of the part increases, which makes the use of the titanium material less meaningful.
Also, a titanium part that does not have to have high abrasion resistance need not be subjected to an oxidation treatment. However, even such a titanium part will also be oxidized when used in a high temperature environment and will eventually have decreased fatigue strength or impact strength too.
In order to overcome those problems, Japanese Patent No. 3151713 discloses a method of attaching aluminum powder on the surface of a titanium valve or a titanium connecting rod by a baking process. Meanwhile, Japanese Patent Application Laid-Open Publication No. 2004-115907 discloses a method of nitrifying a titanium part in a nitrifying chamber. According to each of these methods, the aluminum film or the titanium nitride film that has been formed on the surface of the titanium part functions as an oxygen barrier layer that prevents oxygen from reaching the titanium material as the base material. As a result, the oxidation of the titanium material is minimized and the fatigue strength and impact strength thereof are increased. Also, if the overall titanium part is subjected to an oxidation treatment after such an oxygen barrier layer has been formed on a predetermined portion of the titanium part, then sufficiently high fatigue strength can be ensured where the oxygen barrier layer has been formed and the abrasion resistance can be increased in the other regions (i.e., where a titanium oxide is produced as a result of the oxidation treatment).
According to the methods disclosed in Japanese Patent No. 3151713 and Japanese Patent Application Laid-Open Publication No. 2004-115907, however, good antioxidation effects are not achieved for the following reasons.
Specifically, if the aluminum film is used as an oxygen barrier layer, then a brittle layer of an intermetallic compound (between aluminum and titanium) is formed between the aluminum film and the titanium material layer, which is located inside the aluminum film, during the oxidation treatment or when the titanium part is used in a high temperature environment. Thus, this intermetallic compound layer may have cracks within due to the stress caused by the running engine and the function of the aluminum film as an oxygen barrier layer may decrease as a result. Consequently, good antioxidation effects are not always achieved by such a method. What is worse, the film formed by baking the powder becomes a porous layer that cannot function as a gas barrier layer effectively.
On the other hand, a titanium nitride is so hard and brittle that a titanium nitride film may also have very small cracks due to stress. That is why even if a titanium nitride film is used as an oxygen barrier layer, good antioxidation effects are not always achieved.
Furthermore, the aluminum film and titanium nitride film that have been formed as described above are not dense enough to have high oxygen barrier properties in the first place. For that reason, when exposed to a highly oxidizing gas such as the combustion gas of an engine, the gas will likely pass any of these films to reach and oxidize the underlying titanium material. As a result, a titanium oxide layer will have more and more cracks as the part is used over time. In many cases, eventually the part itself will soon have cracks itself.